It is not a double entendre, since there is one common meaning for "toot" and it is about bodily noises. "Toot" has not been about posting on Mastodon up until very recently.
Do they not have trumpets where you live? Did "tooting one's own horn" imply breaking wind during the annual performance review? Far and away the most common use of "toot" is playing a note on a horn.
"Toot" also has the double entendre. In all my life, I've never heard anyone outside a playground use it to describe flatulence. In my personal experience, it's almost always been about the musical notes. I'm not in a band or otherwise around musicians more than the average person, either.
Using toot in the sense of tooting your own horn is still an odd decision for posts. It implies that users' posts should be focused on bragging about themselves. At least in my opinion that make for a very annoying social media feed to read through.
It's a reference to the trumpeting sound an elephant/mastodon makes, meant as the equivalent to the 'tweet' a bird such as the one in the old Twitter logo makes.
There's nothing more to it than that: A quirky name based on the chosen logo, following the Twitter example.
I agree it wasn't the best word choice, and I can understand why it has been de-emphasised in favour of 'posts', but the reasoning behind it was logical.
